MUSIC PLAYS ONLY ON SPEAKERS, NOT STEREO HEADPHONES
Here's the symptoms:
You have a pair of stereo headphones (2.5MM type) plugged in to your VX8300.
You can hear everything through them EXCEPT music and videos -- those play through the speakers instead.
My problem was that the phone had TTY mode on when I got it -- it behaved normally except that it always sent music to the speakers instead of the headphones. I turned off TTY mode and it was fine.
Go to MENU / SETTINGS AND TOOLS / Call Settings / TTY Mode / TTY OFF.